Electrostatic printing method and apparatus employing corona discharge means



Jan. 3, 1967 K. w. RAREY ET AL 3,295,440

ELECTROSTATIC PRINTING METHOD AND APPARATUS EMPLOYING CORONA DISCHARGE MEANS Filed May 27, 1964 INVENTORS KENNETH wwzmzerr J0HM E ENNEDY United States Patent O ELECTROSTATIC PRINTING METHOD AND AP- PARATUS EMPLOYING CORONA DISCHARGE MEANS Kenneth W. Rarey, South Holland, and John B. Kennedy, Chicago, Ill., assignors to Continental Can Company, Inc., New York, N.Y., a corporation of New York Filed May 27, 1964, Ser. No. 370,610 6 Claims. (Cl. 101114) charged in passing through a stencil having open and closed portions, and thereby deliver such particles onto a substrate which is at a different potential or polarity than that at the stencil.

According to the present invention, uncharged printlng or so-called toner particles are charged and are accelerated by an electrostatic field so they move toward a stencil having conductive portions charged to a different potential or polarity from that at the charging region, and thereby confer upon the particles a momentum by which they move in essentially straight lines toward the stencil and through the open portions thereof and then continue along their paths into contact with the substrate.

Illustrative practices according to the invention are shown on the accompanying drawings, in which:

FIGURE 1 is an upright section through a first form of apparatus according to the invention, with a diagrammatic associate circuit;

FIGURE 2 is an upright section of a second form, with an associate circuit;

FIGURE 3 is a partial upright section of a modified cloud forming structure;

FIGURE 4 is a plan view of a modified corona electrode.

In the practice according to FIGURE 1, a sleeve 10 of insulating material, such as polymethacrylate resin, is mounted on a hollow base 11 which has a transverse reticulate partition 12 above its floor 13' for receiving a supply of toner powder 14. A duct 15 for air under pressure opens into the base, beneath the partition 12. A stencil '16 is placed over the upper end of the cylinder 10. The substrate 20, which is to be printed, is placed above the stencil, being spaced therefrom by support bars 17 of insulating material. At the bottom of the cylinder 10 is a conductive corona ring 21 having pins 22 extending therefrom and into the space to provide corona'elements above the orifice through which powder particles are carried by the air stream upwardly from the supply 14 toward the stencil. A high voltage direct current source 25 has its negative terminal connected by conductor 26 to the corona ring 21 and its positive terminal connected by a conductor 27 to the conductive portions of the stencil 16. Apertures 28 may be provided in the cylinder 10 below the stencil 16; and like apertures may be provided in the bars 17. The cylinder 10 should be sufiiciently larger in cross-sectional area :at the top than the pattern to be printed so that air discharged through the openings 28 does not significantly affect the direction of the rapidly traveling charged toner particles. The cylinder 10* provides a duct in which the charged particles, in the air current, can move: and can be called a corona chamber.

The stencil 16 may be formed of a wire netting, say

of 100 mesh or finer, with parts obscured by imperme able material by known commercial methods.

A suitable toner powder is a commercially available product having pigmented or dyed particles composed of a mixture of 41 percent of n-butyl methacrylate and 59 percent of polystyrene, with an average particle size of about 17 microns. Such toners are nonconductive. Toner particles of conductive material can be employed.

A porous carbon sheet has been found to provide a satisfactory partition 12.

The source 25 delivers a potential difference effective to provoke ionization of the air adjacent the corona elements 22, but insufficient to provoke arcing or sparking along the length of the cylinder I10. In practice potentials of 20 to 60 kilovolts have been employed, with a preferred practice around 35 kilovolts.

With a stencil 16 and a paper substrate 20 in position, and a supply 14 of toner powder, the source 25 is energized and air under pressure supplied to the inlet duct 15. Air flows through the reticulated partition 12, and carries a cloud of particles upward through the orifice and past the corona elements 22. The intense electric field adjacent the elements 22 produces an electric breakdown of the air adjacent these points and a copious supply of ions is produced, both positively and negatively charged. Those having a polarity opposite to that of the corona elements are attracted thereto and discharged. Those having the same polarity as the corona elements are accelerated by the electrical field between the elements 22 and the stencil 16. As the cloud of toner particles passes through this ionized region, the particles accept or adsorb ions and those which thus receive charges of the polarity of the corona elements are accelerated toward the stencil 16, therewith attaining a high velocity before attaining the region of the stencil so that there is rel-atively little deflection of their paths by the opposite charges resident on the stencil wires extending across open portions of the stencil, and such particles continue through and past the stencil and impact upon the substrate 20 and adhere electrostatically thereto. Particles whose paths lead them to closed portions of the stencil are prevented from passing thereto: and largely remain adherent to the stencil when the particle charges have not thus been reversed. It is preferred to remove the adherent particles, before the next printing operation, to attain prints of maximum definition and tone values.

In the practice according to FIGURE 2, the parts are illustratively as before, with inclusion of a backing electrode 30 above the substrate 20; and with inclusion of a voltage divider 31 across the terminals of the source 25. The conductor 26 leads from one terminal to the corona ring 21, and the other terminal is connected by conductor 32 to the backing electrode 30. An adjustable tap 33 on the voltage divider is connected by conductor 34 to the conductive parts of the stencil 16. In the practice according to FIGURE 2, the potential difference between the corona elements and the backing electrode was 35 to 60 kilovolts, with the tap 33 providing a potential difference between the corona elements and the screen 16 of about 30 to 50 kilovolts, wherewith the potential between the screen 16 and the backing electrode was around 5 kilovolts. In the apparatus so used, the chamber 10 provided a distance of about 12 inches between corona elements and stencil, and the backing electrode was supported at from A; to inches above the stencil, noting that non-conductive paper or cardboard substrates in FIG. 12 can be 10 mils or less in thickness. In general, for FIG. 2, prints have been successfully made with to percent of the total applied voltage appearing between the corona elements and the stencil.

The operation is as above, with the corona elements 22 serving to provide electric charges upon the toner particles so that the electrical field between the corona elements 22 and the stencil 16 accelerates the particles as before.

The procedures can be employed in printing upon a conductive or semiconductive substrate. When a metal sheet is being printed, it is preferred to employ the practice of FIG. 2; noting that a separate backing electrode can be dispensed with, and the conductor 32 connected directly to the substrate metal. If the substrate has a bare metal surface to be printed, it is preferred to employ nonconductive toner particles with the stated modified practice of FIGURE 2, to assure maximum retention of the charged toner particles on the oppositely charged substrate by avoidance of discharging of the particles.

The relative polarities of the corona elements and the stencil in FIGURE 1, and of the corona elements, stencil and backing electrode (or metal substrate) in FIGURE 2 may be reversed from those shown, e.g., the corona elements may be positive and the stencil negative.

The velocities imposed by such voltages upon the toner particles caused them to pass the stencil openings and produce satisfactorily defined patterns on the substrate, with the severel practices.

The substrate should be positioned as close to the stencil as feasible. If the substrate is of high electrical conductivity, e.g., a metal sheet or metal laminated paper, contact should be avoided as then the substrate and stencil assume the same potential. With nonconductive substrates, there may be contact. When the substrate has an irregular surface, as with sheets of corrugated board, the practice under FIGURE 2 is preferred, because the momentum of particles at passing the screen can be employed for their continuance into contact with the substrate through the region therebeyond, assisted by the field provided by the backing electrode.

The practice according to FIGURE 1 is valuable when an article to be printed is of such thickness relative to the length of the duct that a backing electrode behind the article is so far away that its effect is minor for further acceleration of particles which have passed the stencil and are moving toward the substrate.

Such a practice is illustrated in FIGURE 1, where the substrate is the side of a closed and sealed cardboard box 40: wherewith a backing electrode above the box would be spaced far from the stencil 16. If the box is filled with cylindrical metal cans of foodstuffs, the situation is even worse, because charges induced on the cans from a backing electrode will be at variable distances from the stencil, due to the shapes and spacings of the cans with a further disturbing effect upon the paths of the particles.

Similar conditions are present when the printing is to be done upon containers having paper bodies, particularly large drums, plastic bottles, and other articles where it is awkward or impossible to provide a backing electrode against the reverse side of the article which is to be printed.

In each form, since the substrate need not be in contact with the stencil, excellent prints may be made upon rough surfaces such as those of nuts and fruits, and upon corrugated board and egg-shell types of paper, without mechanical pressing or mutilation of the surfaces.

An alternative apparatus for generating a cloud of toner particles is shown in FIGURE 3, which can be employed with the other equipment of FIGURES 1 and 2. In FIGURE 3, the duct 10, corona elements 22, stencil 16 and substrate are as before. The bottom of the duct, below the corona elements 22, is sealed to an upwardly divergent cone 45, which in turn is sealed to the downwardly divergent cone 46 which is closed by a spherical floor 47 at its bottom. A duct 48 with a downwardly directed end can deliver a current of air upon and into the supply 14 of toner powder on the floor 47 and form a cloud thereof so that the air stream with the suspended particles move upwardly and through the restricted communication opening 49 between the two cones, and thus pass to the region of the corona elements 22. It is preferred to provide a ball 50 in the upper cone, which acts to seal the opening 49 when no air is entering, being lifted by the air pressure in the lower cone during service and thereby providing an annular passage for the stream. The ball may be a metal head, e.g. a metal bearing ball. The ball serves to establish and maintain a pressure differential between the spaces within the cones, to break up agglomerations of uncharged toner particles, and to assure that the cloud will be dispersed over a relatively large area rather than as a jet from the opening.

The corona device can be comprised of a metal ring 21 with pins 22 extending therefrom, preferably in an upward and inward direction so the cloud flows between the pins. Pins about oneinch in length and secured at their lower ends to the ring and spaced about one inch apart on the circumference of a horizontal circle about 5 inches in diameter have been employed; the pins extending inward and upward toward a point about 4 inches above the center of the circle. Alternatively a grid of fine wire as in FIGURE 4 may be mechanically and electrically connected to the ring 21, e.g., three parallel wires 55, each 3 mils in diameter, spaced about one inch apart and extending across the lower end of the chamber, have been employed.

In each of the embodiments, air pressure was varied over the range from 2 to 16 p.s.i. measured where the air was forced into the apparatus at ducts 15 and 48. At the low end of this pressure range, it was observed that printing speed was low although quality of the printing was satisfactory. At the high end of the range it was observed that the air stream emerging at the top of the apparatus was carrying toner powder with it. Accordingly, it was found most desirable to carry out printing with air pressures of about 5 p.s.i.

While preferred forms and arrangement of parts have been shown, and preferred methods of electrostatic printing have been disclosed in detail herein, it is to be understood that variations in the form and arrangement of parts and in the various method steps may be provided without departing from the spirit and scope of the invention as defined in the appended claimed subject matter.

What is claimed is:

1. An electrostatic printing apparatus comprising a duct having an electrical nonconductive portion, a corona element at one end of the said nonconductive portion, a pattern stencil having open and closed portions and conductive elements, said stencil being located at the other end of the said non-conductive portion, a high voltage direct current source connected to the corona element and the conductive elements for creating electric charges on toner particles adjacent the corona elements and for effecting acceleration of charged toner particles from the corona element toward the stencil, means for introducing an air-borne cloud of toner particles to the region of the corona element, and means for supporting a printable substrate at the side of the stencil remote from the corona element.

2. An apparatus as in claim 1, and comprising a backing electrode, means for supporting said backing electrode in spaced relation to the stencil and for holding the substrate between the stencil and the backing electrode, and a source of direct current for maintaining the backing electrode at the polarity of the stencil and at a higher potential relative to the corona element.

3. An apparatus as in claim 1, in which the means for introducing the cloud comprises a hollow body in communication with the duct at a point adjacent the corona element, a perforated horizontal partition in said body for supporting a supply of toner particles, and an inlet duct for delivering air into said body at a point below the partition.

4. An apparatus as in claim 1, in which the means for introducing the cloud comprises a hollow body in communication with the duct at a point adjacent the corona element and having a cavity, means fer delivering air under pressure into said cavity for effecting air suspension of toner particles present in said cavity, and means at the communication between the body cavity and the duct for controlling the passage of air-borne particles into the duct, said means being responsive to the air pressure of the cavity.

5. The method of electrostatic printing comprising the steps of:

(a) providing a duct having two ends and a non-conductive portion, (b) positioning a stencil having conductive portions and open portions at :one end of said duct, (c) positioning a substrate adjacent to said stencil on the opposite side of said stencil from said duct, (d) introducing an airborne suspension of toner particles at the other end of said duct, (e) locating a corona discharge element in said duct to charge said airborne suspension to toner particles, (f) establishing an electric field between said corona discharge element and said stencil for accelerating said charged particles toward said stencil, some of said particles continuing through said openings in said stencil,

References Cited by the Examiner UNITED STATES PATENTS 1,788,600 1/ 1931 Smyser. 2,725,304 11/ 1955 Land-rigan et al. 2,787,556 4/ 1957 Hass. 2,817,765 12/ 1957 Hayford 961 2,940,864 6/ 1960 Watson. 3,081,698 3/ 1963 Childress et al. 3,161,543 12/1964 Borders et al.

FOREIGN PATENTS 81,920 9/ 1956 Denmark.

ROBERT E. PULFREY, Primary Examiner.

E. S. BURR, Assistant Examiner. 

1. AN ELECTROSTATIC PRINTING APPARATUS COMPRISING A DUCT HAVING AN ELECTRICAL NONCONDUCTIVE PORTION, A CORONA ELEMENT AT ONE END OF THE SAID NONCONDUCTIVE PORTION, A PATTERN STENCIL HAVING OPEN AND CLOSED PORTIONS AND CONDUCTIVE ELEMENTS, SAID STENCIL BEING LOCATED AT THE OTHER END OF THE SAID NON-CONDUCTIVE PORTION, A HIGH VOLTAGE DIRECT CURRENT SOURCE CONNECTED TO THE CORONA ELEMENT AND THE CONDUCTIVE ELEMENTS FOR CREATING ELECTRIC CHARGES ON TONER PARTICLES ADJACENT THE CORONA ELEMENTS AND FOR EFFECTING ACCELERATION OF CHARGED TONER PARTICLES FROM THE CORONA ELEMENT TOWARD THE STENCIL, MEANS FOR INTRODUCING AN AIR-BORNE CLOUD OF TONER PARTICLES TO THE REGION OF THE CORONA ELEMENT, AND MEANS FOR SUPPORTING A PRINTABLE SUBSTRATE AT THE SIDE OF THE STENCIL REMOTE FROM THE CORONA ELEMENT. 